Thymidine and thymine in plasma, urine and CSF is measured by HPLC. Pharmacokinetic parameters are determined in mice and man following high and low doses of thymidine. The purpose of the work is: to determine whether millimolar thymidine known to be cytotoxic to tumor cells in vitro can be attained in vivo; whether such concentrations are toxic to normal tissues; ane to characterize the pharmacokinetic of super concentrations of thymidine.